What is the X in X-mas?
early 15c., from Middle French optique, obtique (c.1300) and directly from Medieval Latin opticus "of sight or seeing," from Greek optikos "of or having to do with sight," from optos "seen, visible," from op-, root of opsesthai "be going to see," related to ops "eye," from PIE *okw- "to see" (see eye (n.)).
optic op·tic (ŏp'tĭk) or op·ti·cal (ŏp'tĭ-kəl)
Of or relating to the eye or vision.
Of or relating to the science of optics or optical equipment.